use https instead of http for u.m.o to avoid redirecting

browser/components/help/locale/en-US/customization.xhtml

@@ -125,7 +125,7 @@ Contributors:
 <h3 id="getting_installing_themes">Getting and Installing Themes</h3>
 
 <p>You can download and install many themes from <a
-  href="http://update.mozilla.org/themes/" target="_blank">Mozilla Update</a>.
+  href="https://update.mozilla.org/themes/" target="_blank">Mozilla Update</a>.
   Internet searches will find many other themes as well.</p>
 
 <p>Usually when you click on a link to install a theme, &brandShortName;
@@ -173,7 +173,7 @@ Contributors:
 
 <p>Installing an extension involves a very similar process to installing
   themes. Extensions can be found at <a target="blank"
-  href="http://update.mozilla.org/extensions/">Mozilla Update</a>.</p>
+  href="https://update.mozilla.org/extensions/">Mozilla Update</a>.</p>
 
 <p>There should be an install link for each extension on that extension's
   page. As long as JavaScript is not disabled in your browser, clicking this